The present invention relates generally to hydraulic lash compensation devices, such as hydraulic lash adjusters (HLA), and more particularly to a hydraulic lash compensation device of the type in which there is both a high pressure chamber and a reservoir or low pressure chamber.
Hydraulic lash adjusters (also sometimes referred to as “lifters”) for internal combustion engines have been in use for many years to eliminate clearance, or lash, between engine valve train components under varying operating conditions to maintain efficiency and to reduce noise and wear in the valve train. Hydraulic lash adjusters operate on the principle of transmitting the energy of the valve actuating cam through hydraulic fluid, which is trapped in a pressure chamber under a plunger. During each operation of the cam, as the length of the valve actuating components varies as a result of temperature changes and wear, small quantities of hydraulic fluid are permitted to enter the pressure chamber, or escape therefrom, thus effecting an adjustment in the position of the plunger, and consequently adjusting the effective total length of the valve train.
The cam operating cycle comprises two distinct events; (1) operation on the base circle and (2) valve actuation. The base circle event is characterized by a constant radius between the cam center of rotation and the cam follower, and during this event, no cam energy is transmitted. The valve actuation event is characterized by a varying radius between the cam center of rotation and the cam follower, which effectively transmits cam energy to open and close an engine valve. During the valve actuation event, a portion of the load resulting from the valve spring, the inertia of valve train components, and cylinder pressure are transmitted through the valve train and through the lash adjuster. The load increases the pressure of the hydraulic fluid within the lash adjuster pressure chamber, in proportion to the plunger area, and in typical hydraulic lash adjusters currently in commercial production, fluid escapes the pressure chamber between the plunger and the wall of the lash adjuster body. Such a device is referred to as a “conventional leakdown” lash adjuster. Although the present invention could be utilized in various types of hydraulic lash adjusters, it is especially adapted for use in an HLA of the conventional leakdown type, and will be described in connection therewith.
There have been proposed lash adjusters which provide “lift loss,” that is, lash adjusters which are capable of shrinking to a certain extent before the sealed high-pressure chamber prevents further movement. Thus, there is a degree of lost motion of the lash adjuster before the valve starts to open. This lost motion is recovered by a spring after the valve has closed. Using such a lash adjuster, a small degree of negative lash can be quickly accommodated by the lost motion of the lash adjuster, thus making it more certain that the valve will close. Previously proposed “lift loss” lash adjusters have required significant modification to more traditional lash adjuster designs and require select fitting of precisely machined components to adjust the desired amount of lift loss.